Can the GPU be used while video editing
No there is no GPU assist for the VSE (sequence editor) and before you ask, no you can't have more cores either. You can make an OpenGL render (without audio IIRC) though. If you look at an OS task manager or resource monitor you will notice that there is very little stressing the CPU or memory. As it is not a core function of Blender it is not well optimized for system resources.